The present invention relates generally to a ridged reflector for use in optical displays and an optical display device incorporating the reflector. More particularly, the present invention relates to a ridged reflector that is capable of yielding preferential viewing cones with selectable shapes for enhanced viewing of back-lighted and reflective liquid crystal displays.
Reflectors are often used in optical displays, such as liquid crystal displays, to permit viewing of the displays in ambient light alone. Prior art reflectors include planar specular reflectors and planar diffusive reflectors. Specular reflectors include a substantially planar surface that is covered with a reflective metallic coating. Specular reflectors are characterized by an angle of incidence being substantially equal to an angle of reflection. Diffusive reflectors typically have a roughened surface which is predominately coated with a metallic reflective coating. Diffusive reflectors are characterized by reflecting and scattering incident light. However, neither prior art specular reflectors, nor diffuse reflectors adequately compensate for the effects of glare in optical displays.
Glare represents an unwanted reflection of incident light off any refractive interface associated with a display device. In practice, the refractive interfaces are generally planar with smooth surfaces that are substantially parallel to one another so that the glare from multiple refractive interfaces may be additive. In general, as the difference between refractive indexes increases at the refractive interface, the amount of reflection also increases from the impedance mismatch at the refractive interface. Glare is characterized in that an incident angle approximately equals the magnitude of a reflection angle. Glare typically occurs at both glancing incident angles and nonglancing incident angles relative to any refractive interface above the liquid crystal material of the display device. Perceived glare is glare which is coincident with or lies within a preferential viewing cone of an optical display. Perceived glare may be perceived by a viewer and may detract from the usable brightness and the legibility of the display. Actual glare may exist regardless of whether or not, it is actually perceived by a viewer.
Glare may be categorized as primary glare and secondary glare. Primary glare occurs as ambient light is reflected from an exterior face of an optical display. Primary glare is typically more prevalent and bothersome to a viewer than secondary glare. Secondary glare occurs as ambient light is reflected from other refractive interfaces within the display without first reaching the reflector. For example, in a twisted nematic display secondary glare occurs when light entering the display is reflected from indium-tin oxide electrodes.
Commercially available glare-reducing films have been used in optical displays to match different impedances at the refractive interfaces so as to reduce glare reflections. The glare-reducing film generally has a thicknesses which is an integer multiple of a quarter wavelength within the visible light frequency range. However, glare-reducing films tend to increase manufacturing costs in a manner which discourages their wide-spread commercial use.
Specular and diffusive reflectors may be further characterized as single mode or dual mode reflectors. Single-mode reflectors merely reflect light. Dual-mode reflectors have both a reflective mode and a transmissive mode. Dual-mode reflectors are sometimes referred to as transflectors. The reflective operational mode is desired when using the device in ambient light. The transmissive mode is desired when using the device in the dark or when inadequate ambient light is present.
A display device has a preferential viewing cone, which defines the relationship between a viewer and a display device, where the viewer has the best vantage of the display based upon observational factors, such as glare, legibility, contrast, and display brightness. The display device typically has a symmetrical viewing cone with a generally circular cross section about an axis normally extending from the display device. The display device has a physical viewing interface, such as a lens or screen.
The preferential viewing cone may not coincide with viewing proclivities of viewers in various circumstances. The viewing proclivities are influenced by human factors which may differ from device to device, incorporating optical displays. For example, cellular phone users may prefer displays which may be brightly viewed by both the driver and the passenger of an automobile. Yet, most commercially available cellular phones do not offer a preferential viewing cone of sufficient horizontal breadth for simultaneously bright viewing by both the driver and the passenger. In another example, seated users of personal digital assistants (PDA's) may prefer displays which may be brightly viewed regardless of the height of the user relative to display, which will further vary with attendant circumstances such as chair heights, table heights, ambient light directivity, and the like. Many commercially available personal digital assistants do not offer sufficient vertical breadth to accommodate seated users of various heights with equally bright displays. As a result, seated users may find PDA's awkward and uncomfortable to use.
Thus, a need exists for a display device which has a viewing cone with a selectable shape corresponding to the viewing proclivities of users, as impacted by human factors and the intended use of an electronic device incorporating the display device. In addition, a need exists for a reflector which reduces perceived glare in display devices.